Linear-motion electro-mechanical transducers, referred to herein as linear transducers, are useful in a number of applications, and are particularly useful in conjunction with reciprocating motion devices such as Stirling cycle machines.
Optimum efficiency is a primary consideration in the design of a linear transducer, with regard to utilization of power, heat, materials, size, and weight. The ratio of energy output to transducer weight is one factor which is particularly important. Minimizing plunger mass is another important consideration, in order to minimize the momentum which must be periodically reversed during machine reciprocation. When a transducer is to be used with a free-piston Stirling engine, axial symmetry around the longitudinal axis of reciprocation may be important to allow the plunger to rotate or spin. Optimum utilization of magnetic fields is another important design consideration, as is preservation of flux source magnetization.
FIG. 1 is a cross-sectional view of a prior art linear transducer 10 which has been found to have an efficient and desirable design. This linear transducer is described by Robert W. Redlich in his U.S. Pat. No. 4,602,174 (FIGS. 4-7 of the Redlich patent). Transducer 10 comprises a high-permeability stator, including an inner stator block 12 and an outer stator block 13. As described in the referenced patent, blocks 12 and 13 can be diametrically opposite one another, duplicated in quadrature, or continuously revolved in a circular, axially-symmetrical embodiment. In each instance, inner stator block 12 has an annular peripheral channel 14 which contains a toroidal electrical coil or winding 16.
Channel 14 is surrounded by a pair of poles 19 and 21 which extend radially outward from inner stator block 12 on either side of electrical winding 16. Poles 19 and 21 have generally cylindrical outwardly-facing surfaces which form a pair of inner pole faces 18 and 20, respectively.
Outer stator block 13 spans inner pole faces 18 and 20. It has an inwardly facing surface which forms an outer pole face 22 opposite to and facing inner pole faces 18 and 20. The inner and outer pole faces have generally complementary diameters, with the diameter of inner pole faces 18 and 20 being smaller than the corresponding diameter of outer pole face 22 so that an annular air gap 23 extends axially through the stator. Inner and outer stator blocks 12 and 13 form a magnetic path around electrical winding 16 as shown by the arrows in FIG. 1.
An annular magnet 26 is positioned to reciprocate longitudinally within annular air gap 23, between the inner and outer pole faces. Magnet 26 has a radially-oriented magnetic polar axis. It has an axial length approximately equal to the axial length of a single inner pole face, and does not axially span the two inner pole faces 18 and 20. As magnet 26 reciprocates axially from one inner pole face to the other, it alternately completes magnetic flux circuits of opposite polarity around coil 16, producing an electric current within coil 16.
While the linear transducer shown in FIG. 1 performs well in many applications, it has several disadvantages. One disadvantage is that magnet 26 must repeatedly travel through an intermediate position, in the "gap" between inner pole faces 18 and 20, in which there is no adjacent high permeability path for magnetic flux. Repeated transitions between high and low permeability conditions eventually degrade magnet performance. Also, the flux source is exposed to opposing fields from other flux sources or other parts of the same flux source.
Another disadvantage of transducer 10 is that any resulting magnetic flux circuit around coil 16 always includes a relatively large air gap between the outer pole face and one inner pole face. This large gap tends to reduce the magnetic flux intensity in the flux circuit, therefore reducing efficiency and limiting peak power output. In addition, the induction in the stator block of transducer 10 is heteropolar, resulting in relatively high specific core losses. This reduces the efficiency of the device.
The invention described below is a linear transducer which addresses the efficiency factors mentioned above and eliminates the noted disadvantages of the prior art.